Conventionally, there has been known a lighting device for a vehicle which uses a semiconductor light emitting device, for example, a light emitting diode (LED) as a semiconductor light source. A lighting control device for controlling an ON operation of the LED is mounted on the lighting device for a vehicle of this type.
The lighting control device includes a switching regulator for supplying a driving current to the LED and control means having a current detecting portion for detecting the driving current, and the control means repetitively supplies/cuts off a voltage to be supplied to the switching regulator upon receipt of a dimming control signal, thereby repeating driving and stopping operations at a high speed. Thus, a dimming operation of the LED is controlled to reduce a mean current of the driving current (for example, see Japanese Patent Document JP-A-2008-198915).
The switching regulator supplies a power of several tens of watts (W) to the LED and converts a source voltage (a battery voltage) into a suitable voltage for a forward voltage of the LED at a high efficiency in order to reduce a circuit loss. An error amplifier of the control means carries out a feedback control over the switching regulator (a lighting control of the LED) in such a manner that the driving current detected by the current detecting portion has a predetermined magnitude. In the feedback control, a reference voltage input to the error amplifier is compared with a voltage converting value of a DC current detected by the current detecting portion and they are controlled to be almost equal to each other.
In the case in which a light emission of a white LED is reduced (extinction), the reference voltage to be input to the error amplifier is lowered to reduce a DC current. When the DC current is lowered, however, there is caused a problem of a color shift (a white color is eliminated).
In order to solve the problem of the color shift, there has been known a PWM (Pulse Width Modulation) extinction method for turning ON/OFF the LED at a high speed (several hundreds hertz (Hz) to several kilohertz (kHz)).
In general, the PWM extinction method is executed by using a circuit configuration in which a current to the LED is limited by a resistor or a circuit configuration of constant current clamping/constant voltage clamping. In the circuit configurations, a switch element is inserted in series between the resistor and the LED, between the constant current clamp circuit and the LED and between the constant voltage clamp circuit and the LED, and ON/OFF operations are repeated at a desirable frequency and duty so that the PWM extinction method is implemented.
In the case in which the PWM extinction method is executed by using a switching regulator, however, an output voltage of the switching regulator is continuously raised to bring a high voltage state while the switch element is OFF when the same switch element as described above is inserted between the switching regulator and the LED. Thus, there is a possibility that a large current might flow to the LED the moment the switch element is turned ON, resulting in a failure of the LED.